principles of radar systems, 2-1 pulsed radar … of a pulsed radar transmitter and receiver. you...

© Festo Didactic 38542-00 93

When you have completed this exercise, you will be familiar with the operating principles of a pulsed radar transmitter and receiver. You will also be familiar with the Radar Transmitter and Radar Receiver of the Radar Training System.

The Discussion of this exercise covers the following points:

Radar Transmitters

Radar receivers

The Radar Transmitter

The Radar Receiver

Radar Transmitters

The purpose of the transmitter in a pulsed radar system is to produce a pulsed RF signal which can be transmitted by the antenna. The RF signal is generated either by a high-power RF oscillator, or a low-power RF oscillator followed by an RF amplifier.

The high-power oscillator converts pulses of dc power directly to pulsed RF at microwave frequencies, as shown in Figure 2-2a. The most commonly used high-power RF oscillator in radar is the magnetron. This is a type of vacuum tube oscillator developed near the beginning of World War II. It is widely used because of its simplicity, ruggedness and efficiency. Its name comes from the fact that it uses a magnetic field to modify the trajectory of electrons in motion.

Figure 2-2b shows a low-power RF master oscillator followed by a power amplifier. The amplifier accepts the low-power RF signal and amplifies it to produce a high-power signal. One common type of high-power amplifier used in radar transmitters is the gridded traveling wave tube amplifier. A control grid inside this tube acts as a modulator and allows a low-power pulse signal to key the amplifier on and off.

A simplified block diagram of the Radar Transmitter is shown in Figure 2-2c. A solid-state RF oscillator produces a low-power RF signal. This signal is not amplified, but is simply modulated by a modulator to produce low-power radar pulses.

Pulsed Radar Transmitter and Receiver

Exercise 2-1

EXERCISE OBJECTIVE

DISCUSSION OUTLINE

DISCUSSION

Ex. 2-1 – Pulsed Radar Transmitter and Receiver Discussion

94 © Festo Didactic 38542-00

Figure 2-2. Generation of radar pulses.

Figure 2-3 shows the waveforms present in a pulsed radar transmitter, where a pulse train is used to modulate a continuous, sinusoidal RF carrier. Typical carrier frequencies for conventional radars range from 220 mHz to 35 GHz. The modulating pulses are rectangular, although somewhat rounded due to bandwidth limitations. The resulting waveform is a pulsed sine wave.



Figure 2-3. Signal waveforms in a pulsed radar transmitter.

The pulse repetition frequency (PRF, or ) is the number of pulses transmitted per unit time. Typical PRF's range from several hundred hertz to several hundred

kilohertz. The interpulse period is equal to .

The pulse width is the pulse duration. It is usually defined as the time interval between the points where the instantaneous value equals 50% of the peak amplitude. Typical pulse widths range from 0.02 s to 60 s, with 1 s being a common value.

The peak power of a pulsed radar signal is equal to the power of the individual pulses (i.e. power when the transmitter is transmitting). The average power is:

where is the average power.

is the peak power.

is the pulse width. is the interpulse period.

The average power can be thought of as the energy per pulse divided

by the interpulse period , or as the peak power multiplied by the duty factor of

the transmitter .

The maximum detection range of a radar is partly determined by the total amount of energy transmitted per unit time, i.e. the average power. To increase detection range, average power can be increased by increasing either the peak power or the pulse width, thus increasing the energy per pulse. As was seen in



Exercise 1-2, however, increasing the pulse width deteriorates the range resolution of the radar. Increasing the PRF, without changing the pulse width, also increases the average power, but for reasons which will be explained in a later volume, decreases the maximum range at which the target range can be accurately determined.

Radar receivers

Most radar receivers operate by detecting the envelope of the received signal in order to recover the original modulating waveform. Envelope detection is illustrated in Figure 2-4. The high frequency carrier is removed from the signal, and only the positive portion of the envelope is retained. The detected pulses are then amplified for further processing and display.

Figure 2-4. Envelope detection.

Envelope-detecting receivers can be divided into two main types: tuned radio frequency (TRF) and superheterodyne. In a TRF receiver, the envelope detection is carried out directly at the RF frequency, as shown in Figure 2-5. This type of receiver is seldom used, since it is generally more costly than a superheterodyne receiver with equal performance.

Figure 2-5. Tuned radio frequency (TRF) receiver.

The most commonly used type of radar receiver is the superheterodyne receiver, shown in Figure 2-6. In this type of receiver, the received signal is mixed with a local oscillator signal. The mixer produces a signal at a frequency equal to the difference between the RF signal frequency and the local oscillator frequency. This intermediate frequency (IF) is much lower than the original RF signal frequency. The IF signal is amplified and filtered by an IF amplifier before the envelope detection takes place.



Figure 2-6. Superheterodyne receiver.

Because the envelope detection takes place at a relatively low intermediate frequency, the superheterodyne receiver is less costly and more flexible than a TRF receiver. Many variations of the basic superheterodyne design are used in radar systems. Often, the RF signal is applied directly to the mixer without amplification, in order to reduce the cost of the receiver. This, however, reduces the sensitivity of the receiver.

In certain radar applications, envelope detection alone does not satisfy the system requirements. In this case, a quadrature detector is often used. This type of detector is capable of detecting the phase of the received signal as well as the amplitude.

Figure 2-7 shows a typical quadrature detector. The input signal is either the RF signal directly from the antenna, or an IF signal. The input signal is divided between two channels, each having a mixer. In both channels, the input signal is mixed with a reference signal from the local oscillator. However, a phase shift is introduced so that the two reference signals are in quadrature (90° out of phase).

Figure 2-7. Quadrature detector.

As the range of a target varies, the amplitude of the detected pulse varies between a positive and negative maximum. This was observed in Unit 1 using the A-scope display. With a quadrature detector, the two output signals are in quadrature. When a pulse in the I (in-phase) channel is at a maximum amplitude, the same pulse in the Q (quadrature) channel is at a null (zero amplitude). If the target range changes slightly so that the pulse in the I channel is at a null, the pulse in the Q channel will be at a maximum, either positive or negative depending on the design of the receiver and the direction of target motion.



The I and Q pulses from a quadrature detector are never at a null at the same time. In many receivers, the I and Q signals are eventually combined to produce a unipolar pulse signal whose amplitude is independent of the phase of the echo signal.

Together, the I and the Q output signals fully represent the phase and amplitude information contained in the received signal. Radar systems using digital signal processing techniques often require both amplitude and phase information. For this reason, quadrature detection is becoming more and more common in modern radar systems. A receiver which detects both the amplitude and phase of the received signal is said to be a coherent receiver.

A superheterodyne receiver translates the received signal to an intermediate frequency. In some receivers, however, the received signal is translated directly to the baseband (dc) without passing through an intermediate frequency. This is accomplished by applying the received RF signal to the mixer(s), and using a local oscillator signal at the same frequency as the RF signal. The mixer produces a signal at a frequency equal to the difference frequency, which is zero (dc), thus recovering the modulating waveform in one step. This type of receiver is known as a homodyne, or DC-IF receiver.


The front panel of the Radar Transmitter is shown in Figure 2-8. The carrier frequency is determined by the FREQUENCY controls in the RF OSCILLATOR section. When set to VAR., the frequency can be adjusted manually from 8 GHz to 10 GHz. In the CAL. position, the carrier frequency is set to a calibrated 9.4 GHz. In the MOD. position, the carrier frequency is modulated according to the FREQUENCY MODULATION controls. Frequency modulation, however, is not used during pulsed operation. At all times, the voltage at the CONTROL VOLTAGE MONITOR OUTPUT is a linear function of the carrier frequency.

The ISOLATOR passes RF power in one direction only. It is used to protect the RF OSCILLATOR from RF power that could be reflected in a backwards direction.

The RF POWER switch allows the RF power to be switched on or off. When in the STANDBY position, no RF power reaches the DIRECTIONAL COUPLER, and the STANDBY LED is lit. When in the ON position, the RF power is passed and the ON LED flashes on and off.



Figure 2-8. The Radar Transmitter.

The DIRECTIONAL COUPLER divides the RF power and sends part of it to the RF OSCILLATOR OUTPUT. This output provides the local oscillator signal for the Radar Receiver. The rest of the RF power is available at the CW / FM-CW RF OUTPUT. The RF power at this output is continuous.

If pulsed operation is desired, the continuous RF power is coupled to the CW RF INPUT of the MODULATOR. The MODULATOR uses pulses received from the PULSE GENERATOR to modulate the RF waveform. The resulting pulsed RF signal is available at the PULSED RF OUTPUT.

The PULSE GENERATOR generates very short pulses which are synchronized with the pulses at the TRIGGER INPUT. The PULSE WIDTH can be set to 1, 2, or 5 ns, or to VARiable. The TRIGGER INPUT signal is a synchronization signal supplied by the Radar Synchronizer.

The Radar Receiver

The front panel of the Radar Receiver is shown in Figure 2-9. This receiver contains a quadrature detector. Since the quadrature detector of the Radar Receiver produces I and Q signals which represent both the amplitude and phase of the received signal, this receiver can be considered to be coherent.

The POWER DIVIDER at the RF INPUT divides the received RF signal, which is then sent to two mixers. The HYBRID JUNCTION divides the LOCAL OSCILLATOR signal into two reference signals which are in quadrature. These reference signals are sent to their respective mixers.

The LOCAL OSCILLATOR signal comes from the RF OSCILLATOR OUTPUT of the Radar Transmitter (see Figure 2-8). This signal is derived directly from the RF signal produced by the RF OSCILLATOR. Since the LOCAL OSCILLATOR signal is at the same frequency as the transmitted and received RF signals, the mixers translate the received RF signal directly to the baseband. Therefore, this receiver is of the homodyne type.

Ex. 2-1 – Pulsed Radar Transmitter and Receiver Procedure Outline


Figure 2-9. The Radar Receiver.

The two POWER DIVIDERS following the mixers divide the mixer output signals to provide the signals required for the various outputs. The PULSED OUTPUT signals are amplified by the two WIDEBAND AMPLIFIERs. The 1-kHz FILTERS, and the CW DOPPLER and FM-CW OUTPUTs are not used in pulsed operation.

The Procedure is divided into the following sections:


Setting up the basic pulsed radar

The Radar Receiver


In this section, you will determine the relationship between the control voltage and frequency of the Radar Transmitter RF OSCILLATOR by measuring the control voltage for various frequencies, and then plotting the relation on a graph. You will also observe the shape of the Radar Transmitter PULSE GENERATOR output signal for various pulse widths, using the Dual-Channel Sampler, and calculate the duty factor of this signal according to the settings made on the Radar Training System. The block diagram of the system used to sample the PULSE GENERATOR output signal is shown in Figure 2-12.

a In this exercise, you are often asked to set the target range so that the amplitude of the target blip observed on the A-scope display is positive and maximum. However, with time, the amplitude of the target blip may vary. This is due to the RF OSCILLATOR of the Radar Transmitter which may experience a slight frequency drift with temperature. To reduce drift to a minimum, it is preferable to let the Radar Training System warm up for at least half an hour before beginning this exercise. If the amplitude of the target blip still varies significantly, slightly readjust the target range as required.

PROCEDURE OUTLINE

PROCEDURE

Ex. 2-1 – Pulsed Radar Transmitter and Receiver Procedure


1. The main elements of the Radar Training System, that is the antenna and its pedestal, the target table and the training modules, must be set up properly before beginning this exercise. Refer to Appendix B of this manual for setting up the Radar Training System, if this is not done yet.

Set up the modules on the Power Supply / Antenna Motor Driver as shown in Figure 2-10.

Figure 2-10. Module Arrangement.

On the Radar Transmitter, make sure that the RF POWER switch is in the STANDBY position.

On the Antenna Controller, make sure that the MANual ANTENNA ROTATION MODE is selected and that the SPEED control is in the 0 position.

Set the POWER switch of the Power Supply to the I (on) position, and then those of the other modules.

2. Connect the CONTROL VOLTAGE MONITOR OUTPUT of the Radar Transmitter to channel 1 of the oscilloscope. This output provides a signal which is identical with that controlling the RF OSCILLATOR frequency.

On the Radar Transmitter, depress the VARiable FREQUENCY push button, then set the RF OSCILLATOR frequency to minimum.

Make the appropriate settings on the oscilloscope to observe the CONTROL VOLTAGE MONITOR OUTPUT signal.

3. On the Radar Transmitter, set the RF OSCILLATOR frequency to 8.2 GHz.

Measure the dc voltage at the CONTROL VOLTAGE MONITOR OUTPUT of the Radar Transmitter, then note the result in the first row of the CONTROL VOLTAGE column of Table 2-1.



Carry out the same manipulations for the other frequencies listed in Table 2-1.

Table 2-1. Control voltage versus frequency for the RF OSCILLATOR of the Radar Transmitter.

CONTROL VOLTAGE FREQUENCY

V dc GHz

8.2

8.6

9.0

9.4

9.8

4. In Figure 2-11, plot the relation between the frequency and control voltage of the RF OSCILLATOR, using the results noted in Table 2-1.

Describe the relationship between the control voltage and frequency of the RF OSCILLATOR. Determine the slope of this relationship.



Figure 2-11. Relation between the control voltage and frequency for the RF OSCILLATOR of the Radar Transmitter.

5. Remove the cable connecting the CONTROL VOLTAGE MONITOR OUTPUT of the Radar Transmitter to the oscilloscope.

Figure 2-12 shows how to connect the Dual-Channel Sampler in order to sample the output signal of the Radar Transmitter PULSE GENERATOR. Connect the modules as shown in this figure.

a Use a medium-length (approximately 75 cm) SMA cable to connect the PULSE GENERATOR OUTPUT of the Radar Transmitter to the I-CHANNEL PULSE INPUT of the Dual-Channel Sampler.



Figure 2-12. Block diagram of the system used for sampling the output signal of the Radar Transmitter PULSE GENERATOR.

6. Make the following adjustments:

On the Radar Transmitter

RF OSCILLATOR FREQUENCY ................... CAL. PULSE GENERATOR PULSE WIDTH .......... 1 ns

On the Radar Synchronizer

PRF MODE ................................................... SINGLE PRF................................................................ 216 Hz

On the oscilloscope

Time Base ...................................................... X-Y Channel X ...................................................... 0.2 V/DIV (DC coupled) Channel Y ...................................................... 0.2 V/DIV (Set to GND)

Set the X- and Y-position controls of the oscilloscope so that the trace is centred on the screen.

Set the Y-channel input coupling switch of the oscilloscope to the DC position. If an offset voltage is present at the I-CHANNEL SAMPLED OUTPUT of the Dual-Channel Sampler, the trace on the oscilloscope screen will shift up or down. If this happens, adjust the I-CHANNEL DC OFFSET control of the Dual-Channel Sampler so that the trace is centred on the oscilloscope screen.

7. On the Dual-Channel Sampler, select the 1.8-m RANGE SPAN, make sure that the GAIN controls are in the CALibrated position, then set the ORIGIN control so that the output signal of the PULSE GENERATOR is centred on the fourth division of the oscilloscope screen.



On the Radar Transmitter, vary the PULSE WIDTH setting of the PULSE GENERATOR while observing its output signal on the oscilloscope screen, then set the PULSE WIDTH to 1 ns. Figure 2-13 shows an example of what you might observe on the oscilloscope screen for various PULSE WIDTH settings.

Figure 2-13. Output signal of the PULSE GENERATOR for various PULSE WIDTH settings, sampled with the Dual-Channel Sampler.



Using the PULSE WIDTH and the actual PRF, calculate the actual duty factor of the pulse signal provided by the PULSE GENERATOR. Recall that the actual PRF is 1024 times the PRF selected on the Radar Synchronizer, as explained in Appendix E.

Setting up the basic pulsed radar

In this section, you will set up a basic pulsed radar and calibrate the A-scope display. The block diagram of this system is shown in Figure 2-14.

8. Remove the SMA cable and the 50 load from the PULSE INPUTS of the Dual-Channel Sampler.

Figure 2-14 shows the block diagram of the basic pulsed radar that can be obtained using the Radar Training System. Connect the modules according to this block diagram.



Figure 2-14. Block diagram of the basic pulsed radar.



9. Refer to Appendix C of this manual to calibrate the A-scope display so that its origin is located approximately 1.0 m from the antenna horn and its range span is equal to 1.8 m. Once you have finished the calibration, the display on the oscilloscope should resemble Figure 2-15.

Figure 2-15. Calibrated A-scope display of a fixed target located at the origin.

The Radar Receiver

In this section, you will observe a target blip on the A-scope display while varying the pulse width on the Radar Transmitter, in order to compare the shape of the target blip with that of the PULSE GENERATOR output signal. You will also observe the I- and Q-CHANNEL PULSED OUTPUT signals of the Radar Receiver simultaneously to determine the phase relationship between these two signals. You will finally observe the role of the reference (local oscillator) signal in the frequency translation of the received RF signal to baseband, by disconnecting the LOCAL OSCILLATOR INPUT signal.

10. On the Target Controller, use the Y-axis POSITION control to place the target at the far end of the target table, then vary the target range by a few millimeters so that the peak voltage of the target blip on the A-scope display is positive and maximum.



On the Radar Transmitter, vary the PULSE WIDTH setting of the PULSE GENERATOR while observing the target blip on the A-scope display, then set the PULSE WIDTH to 1 ns. Figure 2-16 shows an example of what you might observe on the oscilloscope screen for various PULSE WIDTH settings.

Figure 2-16. Fixed target blip for various PULSE WIDTH settings on the Radar Transmitter.



Compare the shape of the target blip with that of the PULSE GENERATOR output signal. Are they alike? Why?

11. On the oscilloscope, disconnect the end of the cable connected to channel X, then connect it to the external triggering input.

Connect the I- and Q-CHANNEL SAMPLED OUTPUTS of the Dual-Channel Sampler to channels 1 and 2 of the oscilloscope, respectively.

Make the appropriate settings on the oscilloscope to obtain a stable display of the I- and Q-CHANNEL PULSED OUTPUT signals of the Radar Receiver. These signals are presently sampled by the Dual-Channel Sampler.

Figure 2-17 shows an example of what you might observe on the oscilloscope screen.

Figure 2-17. I- and Q-CHANNEL PULSED OUTPUT signals of the Radar Receiver.

On the Target Controller, use the Y-axis POSITION control to slowly decrease the target range so that the amplitude of the I-CHANNEL PULSED OUTPUT signal passes from a positive maximum to a negative maximum and then to another positive maximum. While doing this, observe both signals on the oscilloscope screen.

Ex. 2-1 – Pulsed Radar Transmitter and Receiver Conclusion


Describe what you observe on the oscilloscope screen.

Describe the relationship between the I- and Q-CHANNEL PULSED OUTPUT signals.

What is the cause of the phase relationship between the I- and Q-CHANNEL PULSED OUTPUT signals?

12. On the Radar Transmitter, place the RF POWER switch in the STANDBY position.

On the Radar Receiver, disconnect the end of the SMA cable connected to the LOCAL OSCILLATOR INPUT.

On the Radar Transmitter, place the RF POWER switch in the ON position.

Observe the oscilloscope screen. Are there any signals at the I- and Q-CHANNEL PULSED OUTPUT? Why?

13. On the Radar Transmitter, make sure that the RF POWER switch is in the STANDBY position. The RF POWER STANDBY LED should be lit. Place all POWER switches in the O (off) position and disconnect all cables.

In this exercise, you plotted the relationship between the control voltage and frequency of the RF OSCILLATOR. You found that the frequency of the RF OSCILLATOR varies linearly at a rate of 0.25 GHz per volt as the control voltage varies.

CONCLUSION

Ex. 2-1 – Pulsed Radar Transmitter and Receiver Review Questions


You observed that the shape of the target blip resembles that of the PULSE GENERATOR output signal, since the Radar Receiver detects the envelope of the received signal.

You also observed the I- and Q-CHANNEL PULSED OUTPUT signals of the Radar Receiver simultaneously and found that these signals are in quadrature. Finally, you verified that a reference (local oscillator) signal is required to carry out the frequency translation of the received RF signal to baseband.

1. Why is the magnetron the most commonly used high-power RF oscillator in radar?

2. Describe the usual waveform of the transmitted radar signal.

3. How do most radar receivers operate?

4. What is the main advantage of a superheterodyne receiver over a tuned radio frequency (TRF) receiver?

5. What is the main advantage of the quadrature detector?

REVIEW QUESTIONS

principles of radar systems, 2-1 pulsed radar … of a pulsed radar transmitter and receiver. you...

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